In the storage tape drive industry, transmissive optical sensors have been used to measure the dynamic behavior of the edge of the tape traveling on a tape transport subsystem. This technique generally has been used to estimate lateral motion of the tape (LTM) at any location in the tape transport subsystem. The accuracy and sensitivity of this type of measurement, however, is severely impacted by several limitations. Such limitations include imperfection of edge profile that incorrectly registers as LTM, sensitivity limitation due to optical setup, and electro-mechanical and optical noise and drift contamination
Lateral tape motion (LTM) is problematic in tape drives especially as track pitches get smaller and smaller in new generations of magnetic and optical tape drives. Magnetic and optical read/write heads must be servoed to follow track lateral motion to typically better than 1/10th or 1/20th of the track pitch in order to maintain data integrity. As track pitches get smaller, for example, 320 nm in an optical tape drive, LTM measurements accurate to microns or tens of microns are no longer adequate. Precise measurement of LTM is a valuable tool that can aid tape path engineers in minimizing LTM. Having precise tape edge sensors in a production tape drive could be used to reduce LTM by applying feed-forward servo techniques. To measure LTM, optical sensors, such as photonic probes and photo-interrupters, have been used but have characteristics that limit their accuracy and resolution to unacceptable levels compared to the 10 to 20 nm needed for newer, finer track pitches. An important parameter for optical tape edge sensing is the sample length—the length of tape edge “observed” or integrated at any instant. In order to measure LTM without distortions due to tape edge roughness, a longer segment of tape is measured. If tape edge roughness with LTM is to be measured, a shorter segment is measured. Unfortunately, photonic probes and photo-interrupters also have limited ability to change the sampling length of tape.
Accordingly, there is a need for improved methods of measuring lateral tape motion in storage tape drives.